I remember my English professor back in college talking about this subject. It was a confusing idea, but I'll explain what I can. I'm sure I've forgotten a lot.
Back in the day - in the time of the feudal system, is when it started. Remember the overlords would let fiefs use some of their land to live and grow crops on. At first, those poor fiefs had to give service to the overlord. Later it changed and both the fief and the overload had to give something to each other. Only fair,huh?
After feudalism was gone, common law became the way to own land. In this system, the king actually owned all the land, but he would give some to certain people - lucky folks. Then a person who got land from the king could do what he wanted with the land. He could sell it or divide it in fee simple. Hope this makes sense.